In the drilling of an oil well, mud is normally pumped through the drill string, through a specified number of drill collars and through the drill bit to lubricate the hole. As the drill bit penetrates the earth, cuttings from the face of the hole are washed away from the drill bit by mud flowing upwardly in the annulus. As the well deepens, the back pressure of the mud in the annular space increases dependent on the height of the column and the back pressure maintained in the annular system including the choke which delivers the mud to the mud pits. As this back pressure increases, it requires a higher pressure in the drill string to overcome the hydrostatic head of the column of mud standing in the annulus. The high back pressure in the annulus works against the jetting action of the mud flow. This has undesirable results which are a reduction in the rapidity of the washing of rock particles away from the near vicinity of the bit. In addition, it reduces the jetting action. By jetting, reference is made to the tendency of a mud jet flowing from the drill bit to actually wash or erode some of the rock before it has been cut by the teeth on the drill bit. In other words, a portion of the drilling is achieved by the jetting action without regard to the cutting action of the bit itself. Both ramifications are undesirable.
The apparatus of the present invention overcomes these drawbacks. It is necessary of course for the mud to return in the annulus. The mud which flows in the annulus ordinarily may be viewed, statically speaking, as standing on the bottom of the hole itself and thus it interferes with the jetting action and the rapid removal of chips. This is not the case with the present disclosure. Rather, a set of protruding helical turns on a screw mounted on the drill string actually supports the column of mud. More precisely, the mud is picked up by the bottom most flite of the helical thread on the exterior and is pulled rapidly away from the bottom of the well to thereby reduce the pressure just above the drill bit. The helical screw thus provides a type of pumping action and thereby actually enables the column of mud in the annulus to stand on it rather than on the bottom of the hole. It provides an upward pumping action which rapidly evacuates the area near the drill bit, reducing hydrostatic pressure there and initiating the mud in the upward flow. In other words, the apparatus disclosed herein provides a positive pumping action to the mud rather than merely rely on the U-tube effect.
The present disclosure thus is an apparatus which utilizes the energy in the flowing mud stream within the drill string and converts it into rotary motion of a lifting screw on the exterior and immediately above the drill bit to pressurize the mud in the annulus and clear the vicinity of the drill bit.